This invention relates to radio communication systems and methods, and more particularly to systems and methods for transmitting and receiving data on a radio channel.
Public wireless radiotelephone systems are widely used to provide radiotelephone communications to subscribers. For example, the Global System for Mobile communications (GSM) system has been in service since the early 1990""s. The design and operation of the GSM system is well known to those having skill in the art and need not be described further herein.
The GSM system has been extended in order to facilitate wireless packet data communications. In particular, the General Packet Radio Service (GPRS) has been designed to facilitate packet data communications over a radio channel. The GPRS system is described, for example, in European Telecommunications Standards Institute (ETSI) publication GSM 03.60 V.5.2.0 1997-1 entitled Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Service description; Stage 2 (GSM 03.60 version 5.2.0), the disclosure of which is incorporated herein by reference. The design and operation of GPRS is well known to those having skill in the art and need not be described further herein.
Extensions of GPRS, such as Enhanced GPRS (EGPRS) and Enhanced Data Rates for GSM Evolution (EDGE), now are being designed to facilitate high speed communication of multimedia data and packet-based voice, while allowing enhanced compatibility with external network protocols such as the Internet Protocol (IP). The EGPRS and EDGE systems are described in GSM 04.60 V6.2.0 (1998-10) entitled Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)xe2x80x94Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol (GSM 04.60 Version 6.2.0 Release 1997) and GSM 05.03 V8.0.0 (1999-07) entitled Digital Cellular Telecommunications System (Phase 2+); Channel Coding (GSM 05.03 Version 8.0.0 Release 1999), the disclosures of which are hereby incorporated herein by reference. The design and operation of EGPRS and EDGE are well known to those having skill in the art and need not be described further herein.
FIG. 1 is an overall block diagram of a GPRS architecture. As shown in FIG. 1, the GPRS architecture includes a plurality of Mobile Stations (MS) that communicate with the GPRS network using a wireless radiotelephone link. An MS includes a Mobile Terminal (MT) and Terminal Equipment (TE). It will be understood that although the TE and MT are illustrated herein as two separate blocks, they may be implemented using shared components in a single portable housing. The Um access point is used for mobile access and the R reference point is used for origination or reception of messages. An inter-GPRS interface Gp connects two independent GPRS networks for message exchange. The Gi reference point connects the GPRS network to a Packet Data Network (PDN) or other networks. There may be more than a single GPRS network interface to several different packet data or other networks. These networks may both differ in ownership as well as in communications protocol such as X.25, TCP/IP, etc.
FIG. 2 is an overview of a GPRS logical architecture. As shown in FIG. 2, GPRS is logically implemented on a GSM structure through the addition of two network nodes, the serving GPRS Support Node (SPSN) and the Gateway GPRS Support Node (GGSN). The GGSN is a node that is accessed by the packet data network due to evaluation of a packet data protocol address. It contains routing information for attached GPRS users. The SGSN is the node that is serving the MS. At GPRS attach, the SGSN establishes a mobility management context containing information pertaining to, for example, mobility and security for the MS. The MS communicates with a plurality of Base Station Systems (BSS) using a wireless radiotelephone link. Other details of the GPRS logical architecture may be found in GSM 03.60 cited above, and need not be discussed further herein.
FIG. 3 illustrates a transmission plane of a GPRS system. As shown in FIG. 3, the transmission plane includes a layered protocol structure providing user information transfer, along with associated information transfer control procedures such as flow control, error detection, error correction, and error recovery. The transmission plane independence of the network subsystem platform from the underlying radio interface may be preserved via the Gb interface. As shown in FIG. 3, the primary Layer 2 (L2) interface between the MS and the BSS is through the Radio Link Control/Medium Access Control (RLC/MAC) block. The RLC portion offers access to control mechanisms associated with the radio resource. The MAC portion allows access to a physical layer. The transmission plane of FIG. 3 and the RLC/MAC block are defined in the above-cited GSM 03.60 and GSM 04.60.
FIGS. 4A and 4B illustrate a downlink RLC data block with an MAC header and an uplink RLC data block with an MAC header, respectively, for GPRS. The design of the data blocks and the fields therein are defined in the above-cited GSM 04.60 and will not be described further herein.
In providing real-time services such as multimedia and voice under GPRS, it is generally desirable to reduce the protocol related overhead burden introduced by the PDNs or other networks of FIG. 1. Protocol overheads may be reduced by introducing two bearers. The first bearer is called Optimized Voice over EGPRS (OVE), and may be used to offer packet voice services akin to standard telephony and to optimize for maximum spectral efficiency. For the OVE bearer, the only overhead may result from the inband signaling due to the Adaptive Multi-Rate (AMR) vocoder, the RLC/MAC overhead and channel coding. The second bearer is called a General Real-Time Service over EGPRS (GRE), that may be used to offer multimedia services with end-to-end IP connectivity. Such a bearer may be used for applications such as video telephony. The RLC/MAC overhead may be the same as the OVE case. Additional overhead may include a link layer and optional link fields that delineate a plurality of link layer frames.
Accordingly, a major contributor to overhead is the RLC/MAC header that is repeatedly transmitted and received along with data in GPRS/EGPRS on a GSM radio channel. In GPRS/EGPRS, the header and data are coded separately. The performance of the header may be worse than the performance of the payload for at least some of the AMR modes in OVE and for at least some of the coded schemes for GRE bearers. This may lead to loss of performance due to, for example, front-end clipping of speech or video. Accordingly, there is a need to provide reliable decoding of headers without excessively contributing to overhead.
It is known to provide sufficient amounts of channel coding to provide adequate header performance. However, it is also desirable to reduce and preferably minimize the overhead due to channel coding. Thus, for example, the RLC/MAC payload may be coded using as low a rate as 1/12 for the convolutional code while the header may be coded only up to a rate 1/3 in order to maximize the amount of channel coding available for data. Thus, there continues to be a need for systems and methods for reliably decoding an RLC/MAC header without contributing excessively to overhead.
It therefore is an object of the present invention to provide improved systems and methods for transmitting and receiving data on radio channels.
It is another object of the present invention to provide systems and methods for decoding a header that is repeatedly transmitted and received along with data on a radio channel.
It is still another object of the present invention to provide systems and methods that can reliably decode headers without contributing excessively to overhead on the radio channel.
These and other objects may be provided according to the present invention by methods and systems for decoding a header that includes a plurality of header fields and that is repeatedly transmitted and received along with data on a radio channel, by initially decoding at least one initially received header to identify values for the plurality of header fields. The plurality of header fields include invariant header fields having determinate values and changing header fields having changing values. As used herein, invariant header fields generally do not change during a persistent flow communication session of packetized data (including voice, multimedia and/or processed data) whereas changing data fields generally may change with each data packet. At least one subsequently received header is decoded using the determinate values for at least one of the invariant header fields, thereby allowing increased reliability decoding of at least one of the changing header fields. Later, a second header, including the changing header fields but excluding at least one of the invariant header fields, along with data, may be received and decoded, in response to successfully decoding the at least one subsequently received header using the determinate values for at least one of the invariant header fields.
The invention stems from the realization that real-time flow (a persistent flow) that has gained control of a radio channel, such as a time slot in a TDMA frame in a GSM system, is likely to retain control of the radio resource in a continuous manner until the real-time flow proceeds into discontinuous transmissions. Thus, successive uses of the TDMA channel, such as the associated GSM TDMA frame, will likely be from the same real-time flow. Assumptions therefore can be made about the value of invariant header fields during subsequent transmission and reception, to allow improved decoding of the changing header fields based upon these assumptions.
Initial decoding of the at least one initially received header may use predictive decoding to identify soft values for symbols associated with the plurality of header fields. Subsequent decoding can subsequently predictive decode at least one subsequently received header using a probability value of unity or other constrained decoding for symbols associated with at least one of the invariant header fields, thereby allowing increased reliability predictive coding of at least one of the changing header fields. Initial decoding may be repeatedly performed for a predetermined number of times. Alternatively, initial decoding may be repeatedly performed until the values for the plurality of header fields are successfully identified relative to a selection criterion.
The present invention may be particularly advantageous for decoding the Radio Link Control/Medium Access Control (RLC/MAC) header that includes a plurality of header fields and that is repeatedly transmitted and received along with data in the General Packet Radio System (GPRS) on a Global System for Mobile communication (GSM) radio channel. In particular, for the RLC/MAC header, at least one initially received RLC/MAC header is initially decoded to identify values for the plurality of header fields. The plurality of header fields include invariant header fields having determinate values and a Coding and Puncturing Scheme (CPS) header field having changing values. At least one subsequently received header is then subsequently decoded using the determinate values for at least one of the invariant header fields, to thereby allow increased reliability decoding of the CPS header field by constrained decoding candidate selection. In the RLC/MAC header, a Power Reduction (PR) field also may have changing values, albeit very rarely, so that during subsequent decoding at least one subsequently received header is decoded using the determinate values for at least one of the invariant header fields, to thereby allow increased reliability decoding of both the CPS and PR fields. During this decoding, the PR field may be treated as invariant, to thereby allow increased reliability decoding of the CPS field. As was described above, predictive decoding may be used and initial decoding may be repeatedly performed for a predetermined number of times or until the values for the plurality of header fields are successfully identified.
According to another aspect of the present invention, headers are decoded by repeatedly transmitting a first header including invariant header fields and changing header fields, along with data, over a radio channel during a persistent flow communication session. The first header, including the invariant header fields and the changing header fields, is decoded and an indication may be provided that the first header, including the invariant header fields and the changing header fields, has been decoded successfully. Upon successful decoding of the first header, a second header is repeatedly transmitted that includes the changing header fields but excludes at least one of the invariant header fields. The second header, including the changing header fields but excluding at least one of the invariant header fields, then is decoded.
This aspect of the invention stems from a realization that once the invariant fields of the header have been decoded successfully, they need not be repeated in subsequent headers. Accordingly, a new header may be used that need only include the changing header fields but that can exclude at least one of the invariant header fields. The same coded header length as the first header may be employed so that the rate of coding can increase and allow improved reliability of decoding the changing header fields. Thus, the first header and the second header preferably have the same coded length, wherein the first header is decoded at a first decoding rate and the second header is decoded at a second decoding rate that is higher than the first decoding rate. At least one stealing bit may be used to indicate transmission of the second header rather than the first header.
This aspect of the invention also may be applied to the RLC/MAC header by repeatedly transmitting a first RLC/MAC header including invariant header fields and a CPS header field having changing values, along with data, over the GSM radio channel during a persistent flow communication session. The first header, including invariant header fields and the CPS header field, is decoded. An indication of successful decoding may be provided. A second RLC/MAC header, including the CPS header field but excluding at least one of the invariant header fields, along with data associated with the persistent flow communication session, then is repeatedly transmitted over the GSM radio channel in response to the indication. The second RLC/MAC header then is decoded. The second RLC/MAC header also may include the PR header field as well as the CPS header field. As was described above, the PR header field may be treated as being invariant in the second RLC/MAC header. Accordingly, reliable decoding of headers may be provided without the need to contribute excessively to overhead.